"(Science: zoology) Walking on the sole of the foot; pertaining to the plantigrades.
Having the foot so formed that the heel touches the ground when the leg is upright.
Origin: L. Planta sole of the foot _ gradi to walk: cf. F. Plantigrade.
(Science: zoology) A plantigrade animal, or one that walks or steps on the sole of the foot, as man, and the bears."
Link#1
http://www.biology-online.org/dictionary/plantigrade
heres something prehaps a bit more at your level.
"Did you know that you are a plantigrade creature, as are such animals as porcupines and bears? Or that a bobcat is digitigrade and a horse unguligrade? No, these words aren't commenting on moral status or astrological sign. These are biological terms referring to the construction of feet and how they meet the turf.
Plantigrade means that the whole foot--heel to toes--meets the ground. Digitigrade refers to toes and fingers--the digits. Thus digitigrade animals, like our desert foxes and rodents, walk on their toes, with the heel of the foot well off the ground. This lengthens the legs, giving them four segments to each limb instead of our three. Our Pronghorn and deer carry things yet further. They are unguligrade, referring to walking on the very tip of the toes, giving them what some people think is a backwards-facing knee--actually the heel, far up the leg. The knee is much higher and almost concealed. The extreme lengthening helps adapt these animals for fast running--for a cursorial way of life"
and the link
http://museum.utep.edu/archive/mammals/DDplantigrade.htm